Did a 9th human exist in the Underground?

A few days ago, I watched this video:

"https://youtu.be/2wEqNpDoRrA?si=VrxWHO-a1jMBgpds"

The theory itself isn’t important for this post (if you're interested, you can watch it on your own). What caught my attention is that, at one point, the creator of the video says that it seems like the generator in the True Lab (the one that gets turned on to restore power to the elevator before all the Amalgamates try to attack you) contains a human soul, powering the entire structure with determination. And after taking another look at the generator, I noticed it too.

But now I wonder— is it just a coincidence? (Considering Toby, it probably is.) Or, if it really is a human soul, whose is it?

It can't belong to the six humans who fell before Frisk, since Asgore has their souls. The soul’s color is red, so maybe it’s Chara’s. But I don’t think Asgore would have allowed Alphys to use one of his "children’s" souls as a battery. Plus, Toriel buried Chara in the Ruins, so it’s unlikely that Alphys went all the way there to take it.

So, whose is it? I don’t know— I’ve been thinking about this for days, and I just can’t figure it out...